1. Field of the Invention
The present invention relates generally to fail safe braking devices for rotating wheels, shafts, and the like, and more specifically to a brake adapted for use with large electric motors as used in overhead cranes and the like. The present brake is automatic, in that if power is lost to the brake, the brake is automatically applied to stop rotation of the motor to prevent "block drop," i.e., to prevent a hoist block and/or any articles suspended therefrom, from falling if the motor is left to turn freely.
2. Description of the Related Art
Overhead cranes, i.e., cranes operating from a lateral beam or the like which is suspended from parallel tracks at both ends, are commonly used in heavy industry for moving heavy or bulky components from one point to another in a factory or plant. Such cranes typically have a large electric motor movably positioned along the lateral beam, thus providing two horizontal degrees of freedom of motion for a hoist block suspended from the motor. The hoist block typically suspended from such motors is generally quite large and heavy, in order to provide the required mechanical advantage for lifting heavy objects.
While the hoist motors themselves are almost universally geared to provide greater torque and to reduce output speeds to a reasonable level for lifting and lowering massive objects, it is nevertheless sometimes possible for the motor to be driven by a massive object suspended from the hoist block of the motor, when no power is being delivered to the motor for whatever reason. This is called "block drop," when the hoist block (and any object suspended from the block) falls essentially freely from the overhead motor.
This is an extremely hazardous situation, and accordingly, various automated motor braking devices have been developed to stop the motor in the event that electrical power is lost to the motor. These brakes generally include heavy springs which urge the brake shoes to an actuated position against a drum which rotates with the motor or transmission output shaft, with an electric solenoid having electrical power applied thereto to cause the springs to be retracted. The solenoid is generally wired in the same circuit as the motor, so when power is lost to the motor, the solenoid also loses power, thereby allowing the springs to clamp the brake shoes against the brake drum to prevent the motor from turning freely.
While the above principle works reasonably well, it is essential that the brake mechanism receive frequent inspections and service to ensure that it will work well when needed. The present invention includes various improvements over devices of the related art, to reduce the service and maintenance normally required of such devices. A discussion of related art known to the inventor, and its differences from the present invention, is provided below.
U.S. Pat. No. 3,348,639 issued on Oct. 24, 1967 to Victor H. Ames et al. describes an Industrial Brake having an automatic adjuster to take up slack as the brake shoes wear. The need for adjustment of slack in such systems is well known, as excessive travel of the solenoid plunger within the solenoid, due to excessive wear of the brake shoes without adjustment, can cause the solenoid coils or windings to burn out. Such solenoids are extremely costly to replace, and Ames et al. have developed a threaded, rotary adjuster to maintain a minimum amount of slack or travel in the system. The present invention differs in that no automatic adjustment is provided or required, as the linkage does not allow sufficient slack to develop as to endanger the solenoid windings. Also, the solenoid of the present automatic brake is offset from the lateral axis of the brake shoes, enabling the brake shoes to be removed and replaced without need to remove the solenoid and thus affect the adjustment.
U.S. Pat. No. 4,020,926 issued on May 3, 1977 to John R. Slatter et al. describes a Brake Apparatus Including Pressed Or Stamped Components. The primary object of the device is to provide an economically manufactured brake, and as such, the device lacks many features of the present brake. Slatter et al. require adjustment for their brake linkage, use a hinged solenoid arm rather than an axially moving plunger, and place their solenoid in line with the lateral axis of the brake shoes, thus requiring removal of the solenoid and adjustment of the linkage when at least the adjacent brake shoe is removed and/or replaced.
U.S. Pat. No. 4,116,308 issued on Sep. 26, 1978 to Janez F. Sever describes an Electromagnetically Operated Friction Brake having a laterally offset solenoid aligned with the brake shoe lateral axis, unlike the present invention. Sever utilizes an excessively large solenoid having little mechanical advantage, but still requires periodic adjustment to compensate for wear in the brake shoes or linings. While Sever provides a manual lever to spread the brake shoes in the event that no power is provided to actuate the solenoid, the present motor brake is easily manipulated to release the brakes without power to the solenoid, merely by applying pressure on the solenoid plunger in the manner in which the solenoid is normally electrically actuated.
U.S. Pat. No. 4,196,797 issued on Apr. 8, 1980 to David B. Taylor describes a Braking Apparatus And Control which selectively limits the rotational motion of the brake assembly about a drum or shaft which it is braking. The brake actuator operates oppositely to that of the present invention, in that the actuator extends to release the brake, rather than retracting, as in the present invention. Also, the actuator is a hydraulic strut, rather than an electric solenoid, as used in the present invention. The Taylor brake is considerably more complex than the present invention, in that it uses electronics to control the flow of hydraulic fluid to various components. Also, the brake actuator appears to be aligned with the lateral axis of the brake shoes, with the removal and/or replacement of the shoes requiring major disassembly of the device, unlike the present invention.
U.S. Pat. No. 4,787,484 issued on Nov. 29, 1988 to David J. Webb describes an Electromagnetically Operated Friction Brake Having Automatic Adjustment Mechanism. The basic mechanism is quite similar to that of the Sever '308 patent discussed further above, in that a relatively large, horizontally acting solenoid is used to operate the brake shoes. The solenoid is apparently aligned laterally with the brake shoes, unlike the configuration of the present invention. Webb discloses an automatic adjuster using a one way clutch operating on a threaded rod, while the present invention is free of any need for such automatic adjustment means
U.S. Pat. No. 4,875,558 issued on Oct. 24, 1989 to Dietmar Berkhan et al. describes a Safety Brake For Escalators, wherein the brake comprises a disc with a grooved circumference with mating shoes or pads fitting tightly within the disc groove when the brake is actuated. The brake is actuated when electrical current to a solenoid is terminated, allowing the solenoid to extend and the brakes to be actuated by spring pressure, somewhat similarly to the operation of the present brake. However, the Berkhan et al. brake solenoid is aligned with the plane of the disc and lateral axis of the brake shoes, unlike the present brake configuration. Also, the present brake incorporates different locking means for maintenance than the spring loaded pawl arrangement of the Berkhan et al. brake.
U.S. Pat. No. 5,337,878 issued on Aug. 16, 1994 to Martin Mehlert et al. describes an Assembly And Method For Adjusting Brake Force for Passenger Conveyor Emergency Brake. The specific type of brake is unclear, as a brake disc (rather than drum) is shown and described, but brake shoes operating against the edge of the disc are also shown and described. The device incorporates a controller to adjust the rate of brake application, depending upon the load on the system. This is beyond the scope of the present invention, which applies the brakes consistently whenever electrical power is removed from the solenoid. The Mehlert et al. solenoid is aligned with the plane of the disc and the lateral axes of the brake actuating arms, unlike the arrangement of the present invention.
British Patent Publication No. 521,825 accepted on May 31, 1940 to Ernest R. Elliston describes Improvements In Or Relating To Electro-mechanical Brakes. The system uses a pneumatic damping cylinder with the brake actuation spring therein and weight on the outer portion thereof, so that when electrical current to the solenoid is terminated, the spring is restricted from rapid movement due to the pneumatic action of the cylinder. Brake application is thus a gradual procedure. The present invention does not incorporate any such means of restricting initial brake application, and moreover the present brake includes various other features not disclosed by the Elliston British patent, such as the laterally offset solenoid, lack of any requirement for adjustment, and other features.
A publication by the General Electric Company (no date given) describes the CR-9510 Automatic Solenoid Brakes manufactured by that company. The various brakes shown each have the solenoid positioned in line with the lateral axis of the brake shoes, along the diametric center of the shoes, unlike the laterally offset solenoid of the present invention.
Finally, a publication by the Victor R. Browning & Co., Inc. (no date given) describes various features of their AC-63 to AC-135 and SW motor brakes. The publication essentially describes various adjustment features of the Browning brakes, none of which are used in the present invention. It is also noted that, as with all of the other brakes of which the present inventor is aware, the solenoids of the Browning Brakes are aligned with the lateral axis of the brake shoes in each case, whereas the present brake solenoid is laterally offset to provide for ease of removal and replacement of the brake shoes.
None of the above inventions and patents, either singly or in combination, is seen to describe the instant invention as claimed.